The popularity of photography is continuously increasing. This applies especially to digital photography as the supply of inexpensive digital cameras has improved. Also the integrated cameras in mobile phones have contributed to the increase in the popularity of photography.
Digital imaging and image manipulation have many advantages over conventional film photography. Digital images can be archived and manipulated electronically and the digital form of images offers several possibilities.
Digital imaging devices utilize an imaging sensor, which is a light sensitive device. An imaging sensor detects light and outputs an electrical current which is in proportion to the detected light. Sensors are typically based on silicon technology. A problem with imaging sensors is that it is sensitive not only to visible light but also to infrared radiation. Infrared radiation, or infrared light, is typically defines as light having a wavelength longer than 780 nm. Infrared light distorts images in daytime conditions. Color balance in color images or the balance of dark and light areas in black and white imaging distorts if infrared light reaches the imaging sensor. Thus, in daytime conditions infrared light is blocked from reaching the imaging sensor by placing an infrared blocking filter in front of the imaging sensor.
However, sometimes in dark conditions, when there is very little or no visible light, it is possible to take images by utilizing infrared light. By utilizing an external infrared light source it is possible to take images even without the subject of imaging being aware of the imaging process. This may be advantageous especially in observation cameras, which are used as burglar alarms, for example.
Thus, imaging devices should block infrared light from reaching the sensor in daytime conditions but yet enable infrared imaging in dark conditions. In prior art solutions, a removable infrared block filter or a leaky infrared filter has been used. An infrared filter may be a separate part in an objective of a camera. In dark conditions, the infrared filter may be mechanically removed in front of the objective or the sensor of the camera. This removal may be realized automatically or manually. A drawback of this solution is that it is quite expensive and the long term durability is uncertain because of the moving parts required in the solution. In some infrared filter solutions, some of the infrared light is passed to the sensor. For example, when an external infrared light source is used in dark conditions, the infrared filter is designed to pass the wavelength of the light emitted by the light source through but block all other wavelengths. This solution is a compromise in both daylight and dark conditions and the resulting images are not optimal.